This invention relates in general to a method for a semiconductor ribbon-to-ribbon conversion process and, more specifically, to a method for a rigid edge mode semiconductor ribbon-to-ribbon conversion process.
In the manufacture of semiconductor devices such as transistors, integrated circuits, photovoltaic devices, and the like, the semiconductor industry uses large quantities of semiconductor material, usually silicon, in the form of thin wafers or sheets. It has been conventional to produce the semiconductor wafers by first growing a single crystal semiconductor ingot, sawing the ingot into a plurality of thin slices, and then lapping and polishing the slices to the desired thickness and surface finish. While this process has proved satisfactory for most semiconductor devices, it is too expensive for some large area semiconductor devices and especially for large area photovoltaic devices or solar cells. In fact, in order that photovoltaic devices become a viable alternative energy source, a significant reduction in the cost of the semiconductor starting material is essential.
One technique which has been proposed and developed for the production of thin sheets of semiconductor material suitable for the production of solar cells is the so-called ribbon-to-ribbon (RTR) growth process. In this process, a polycrystalline ribbon is transformed directly into a macrocrystalline ribbon without the need for the costly processing of large diameter ingots. The RTR process uses one or more scanned beams of energy impinging on a polycrystalline ribbon to locally melt the ribbon and to induce crystal growth as the ribbon is translated past the energy beam. As the molten zone moves along the ribbon, the material behind the zone resolidifies in a macrocrystalline form. The macrocrystalline structure is one in which the crystals are of sufficiently large size to permit efficient semiconductor action. Therefore, a monocrystalline ribbon wherein the ribbon is but a single crystal is encompassed within the term "macrocrystalline".In this context, the word "ribbon" generally implies a long strip or sheet having a width much greater than its thickness. Typical dimensions might be a length of up to several meters, a width of 10-100 millimeters and a thickness of 50-250 micrometers.
There are a number of difficulties, however, which are incumbent upon the ribbon-to-ribbon conversion process. To control and stabilize the melt, for example, it is necessary that the molten zone be of limited extent and not extend to the edge of the ribbon. Thus the process is typically carried out in the rigid edge mode wherein edge portions of the ribbon are not melted but are instead left intact. By maintaining a rigid edge the molten zone is always bounded; without the rigid edge the molten zone, under the influence of surface tension, tends to "neck in" making it extremely difficult to maintain the desired width of the macrocrystalline ribbon. In the extreme, the molten zone may collapse, terminating growth. As heretofore practiced, the rigid edge mode is achieved by limiting the scan of the beams to the interior regions of the ribbon, or by gating the impinging energy beam using sophisticated electronic equipment. These techniques are complicated and limit the flexibility of the process, especially when multiple ribbons are grown simultaneously.
A further difficulty results from non-uniformity in thickness or from breaks in the polycrystalline source material. Such non-uniformities result in loss of control of the molten zone and can further result in the separation of the polycrystalline and macrocrystalline ribbon segments. The possibility of the segments separating requires polycrystalline and macrocrystalline materials be independently supported even though such support would not be required in the ideal rigid edge mode process.
The kinetics of the growth process lead to a preference for a molten zone having a particular shape. One such shape, for example, referred to as the "dogbone" melt zone shape because of the enlarged end portions of the molten zone, provides for growth vectors at the freezing boundary to be directed away from the ribbon edge. The desired shape can be achieved through further control and gating of the energy source, but such shaping adds to the difficulties of controlling the beam in the rigid edge mode.
A further difficulty results simply from the fragile nature of the ribbon being transformed. The fragile ribbon is susceptible to breakage and requires careful handling. Such requirement adds to the complexity of the mechanical system required for translating the ribbon past the energy source.
Accordingly, a need existed to develop an improved process which would overcome the above mentioned and other problems of prior art processes to provide an efficient, low cost semiconductor ribbon.
It is therefore an object of this invention to provide an improved process for the ribbon-to-ribbon conversion of polycrystalline to macrocrystalline semiconductor material.
It is a still further object of this invention to provide an improved process for the rigid edge conversion of polycrystalline ribbon to macrocrystalline ribbon.